What is the difference between the two types of outlets?
Modern, grounded 120-volt receptacles, also referred to as outlets, in North America have a small, round ground slot centered below two vertical hot and neutral slots, and it provides an alternate path for electricity that may stray from the appliance.
Since the early 1960s, most electrical codes have required a ground wire to be run to all outlets and appliances. Prior to that time, most wiring was just two wires (hot and neutral). The ground wire provides an alternate path for electricity that may stray from an appliance or product to make its way safely back to the breaker or fuse box and exit the building into the main ground connection.